1. Field of the Invention
The present invention relates to a method of producing a semiconductor film having an amorphous structure relying upon a plasma CVD method, to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs) using the semiconductor film, and to a method of their production. The invention relates to an electro-optical device as represented by, for example, a liquid crystal display panel and to an electronic device mounting such an electro-optical device as a component.
In this specification, the semiconductor device stands for devices that work by utilizing semiconductor characteristics as a whole. Therefore, electro-optical devices, semiconductor circuits and electronic devices are all semiconductor devices.
2. Description of the Related Art
There has heretofore been known a thin-film transistor (hereinafter referred to as TFT) as a typical semiconductor device using a semiconductor film having a crystalline structure. While the TFT is drawing attention as a technology for forming an integrated circuit on an insulating substrate such as of a glass, the liquid crystal display device of the type integral with a drive circuit has now been put into practical use. So far, the semiconductor film having a crystalline structure has been prepared by subjecting the amorphous semiconductor film deposited by plasma CVD method or by a reduced-pressure CVD method to the heat treatment or to the laser-annealing method (technology for crystallizing the semiconductor film by the irradiation with a laser beam).
The semiconductor film having the crystalline structure thus prepared is an aggregate of a number of crystalline particles, and their crystal azimuths are oriented in arbitrary directions and are not controllable, serving as a factor of imposing limitation on the TFT characteristics. In order to cope with the above problem, Japanese Patent Laid-Open No. 7-183540 discloses a technology for preparing a semiconductor film having a crystalline structure by adding a metal element such as nickel that assists the crystallization of a semiconductor film, making it possible not only to lower the heating temperature necessary for the crystallization but also to enhance the orientation of the crystal azimuth in one direction. When a TFT is formed by using the semiconductor film having such a crystalline structure, not only the electric field mobility is improved but also the sub-threshold coefficient (S-value) decreases, and the electric characteristics are strikingly improved.
Use of a metal element that assists the crystallization makes it possible to control the generation of nuclei during the crystallization. Therefore, the film quality becomes homogeneous compared to those obtained by other crystallization methods which permit nuclei to generate in a random fashion. Ideally, it is desired to completely remove the metal element or to a permissible range. With the metal element being added to assist the crystallization, however, the metal element remains in the inside or on the surface of the semiconductor film having the crystalline structure, becoming a cause of dispersion in the characteristics of the elements that are obtained. For instance, the off current increases in the TFT, arousing a problem of dispersion among the individual elements. That is, the metal element for assisting the crystallization turns out to be rather unnecessary after the semiconductor film having the crystalline structure has been formed.
Gettering using phosphorus is effectively utilized as a method of removing the metal element that assists the crystallization from a particular region of the semiconductor film that has the crystalline structure. For example, upon conducting the heat treatment at 450 to 700° C. while adding phosphorus to the source/drain region of the TFT, the metal element can be easily removed from the channel-forming region.
Phosphorus is injected into the semiconductor film having the crystalline structure by the ion-doping method (a method in which PH3 and the like are dissociated with a plasma, and ions are accelerated in an electric field so as to be injected into the semiconductor without, however, separating the ions by mass). For effecting the gettering, however, the phosphorus concentration must not be lower than 1×1020/cm3. Addition of phosphorus by the ion-doping method causes the semiconductor film having the crystalline structure to become amorphous, while an increase in the phosphorus concentration hinders the subsequent recrystallization by annealing. Further, phosphorus added at a high concentration brings about an increase in the treatment time needed for the doping, arousing a problem of decrease in the throughput in the doping step.
Further, the concentration of boron for inverting the type of electric conduction must be 1.5 to 3 times as great as that of phosphorus added to the source/drain region of the p-channel TFT, bringing about a problem of an increase in the resistance in the source/drain region accompanied by a difficulty in effecting the recrystallization.
When the gettering is not sufficiently conducted and becomes irregular in the substrate, a difference or dispersion occurs in the characteristics of the TFTs. In the case of the transmission-type liquid crystal display device, a dispersion in the electric characteristics of the TFTs arranged in the pixel portions turns out to be a dispersion in the voltage applied to the pixel electrodes, whereby a dispersion occurs in the amount of light transmitted which is, then, perceived by the eyes of the viewer as the shade in the display.
For the light-emitting device using OLEDs, TFTs are indispensable elements for realizing the active matrix drive system. Therefore, the light-emitting device using OLEDs must have at least TFTs that work as switching elements and TFTs for feeding a current to the OLED in each of the pixels. Irrespective of the circuit constitution of the pixel and the driving method thereof, the brightness of the pixel is determined by the on current (Ion) of the TFT that is electrically connected to the OLED and feeds the current to the OLED. Therefore, when white is displayed on the whole surface, dispersion occurs in the brightness unless the on current is maintained constant.
This invention is concerned with means for solving the above problems, and provides a technology for effectively removing the metal element remaining in the film after the semiconductor film having the crystalline structure is obtained by using the metal element that assists the crystallization of the semiconductor film.
The gettering technology is occupying a position as an important technology in the production of integrated circuits by using a single crystalline silicon wafer. Gettering is a technology in which metal impurities taken in by the semiconductor are segregated to a gettering site due to some energy, whereby the impurity concentration is lowered in the active region of the element. Gettering can roughly be divided into two; i.e., extrinsic gettering and intrinsic gettering. The extrinsic gettering brings about the gettering effect by applying a distorted field or a chemical action from the outer side. This can be represented by the gettering by which phosphorus ions of a high concentration are diffused from the back surface of a single crystalline silicon wafer. The above-mentioned gettering using phosphorus can be regarded to be a kind of the extrinsic gettering.
On the other hand, the intrinsic gettering is the one which utilizes the distorted field of lattice defect caused by oxygen formed in the single crystalline silicon wafer. This invention is based on the intrinsic gettering that utilizes the lattice defect or lattice distortion, and employs the following means for being adapted to the semiconductor film having a thickness of about 10 to about 100 nm and having a crystalline structure.
This invention comprises the steps of forming a first semiconductor film having a crystalline structure on the insulating surface by using a metal element that assists the crystallization of a semiconductor; forming a film (barrier layer) that serves as an etching stopper on the first semiconductor film; forming a second semiconductor film (gettering site) containing a rare gas element on the barrier layer; gettering the metal element into the gettering site; and removing the second semiconductor film.
In the step of forming the gettering site according to the present invention, a film is formed by the plasma CVD method by using a monosilane, a rare gas element and hydrogen as starting gases, and the film is used as a semiconductor film or, typically, as an amorphous silicon film containing the rare gas element at a high concentration and having an amorphous structure. It is also allowable to use a disilane or a trisilane instead of the monosilane. The plasma CVD method is capable of cleaning the interior of the film-forming chamber (also called chamber) with a gas, requires less maintenance than the sputtering method and is, hence, a film-forming method adapted to mass production.
Besides, this method forms the film by using hydrogen as one of the starting gases and, hence, hydrogen is contained at a decreased concentration in the film as compared to that of when hydrogen is not used as one of the starting gases. As compared to when hydrogen is not used as one of the starting materials, further, fluorine is contained in the film at a decreased concentration since the film is formed by using hydrogen as one of the starting gases.
A method of producing a semiconductor film constituted according to this invention and disclosed in this specification comprises introducing a monosilane, a rare gas and hydrogen as starting gases into a film-forming chamber, generating a plasma, and forming, on a surface on which the film is to be formed, a semiconductor film containing a rare gas element at a concentration of 1×1018/cm3 to 1×1022 cm3 and having an amorphous structure.
In generating the plasma in the above constitution, it is desired that the pressure in the film-forming chamber is from 2.666 Pa to 133.3 Pa and, desirably, smaller than 53.32 Pa (0.4 Torr).
In the above constitution, further, the ratio of flow rate of hydrogen to the rare gas (H2/rare gas) is controlled to be from 0.2 to 5.
In the above constitution, further, the RF power density for generating the plasma is from 0.0017 W/cm2 to 1 W/cm2. When the RF power is not smaller than 1 W/cm2, the film becomes defective, such as becoming powdery or forming semispherical bubbles on the film surface.
In the above constitution, further, the monosilane, the rare gas element and hydrogen are used as starting gases being controlled at a ratio (monosilane:rare gas) of from 0.1:99.9 to 1:9 and, preferably, from 1:99 to 5:95 to form the film thereby to obtain a semiconductor film or, typically, an amorphous silicon film containing the rare gas element at a high concentration and having an amorphous structure. It is further allowable to use a disilane or a trisilane instead of the monosilane. The temperature for forming the film is preferably from 300 to 500° C.
In the above constitution, further, the fluorine concentration in the semiconductor film is from 2×1016/cm3 to 8×1016/cm3 and, preferably, from 1×1015/cm3 to 1×1017/cm3.
A method of producing a semiconductor device constituted according to this invention and disclosed in this specification comprises a first step of forming a first semiconductor film having an amorphous structure on an insulating surface; a second step of adding a metal element to the first semiconductor film having the amorphous structure; a third step of forming a first semiconductor film having a crystalline structure by crystallizing the first semiconductor film; a fourth step of forming a barrier layer on the surface of the first semiconductor film having the crystalline structure; a fifth step of forming a second semiconductor film containing a rare gas element on the barrier layer by a plasma CVD method; a sixth step of removing or decreasing the metal element in the first semiconductor film having the crystalline structure by gettering the metal element into the second semiconductor film; and a seventh step of removing the second semiconductor film.
In the above constitution, the second semiconductor film is formed by plasma CVD method that generates plasma by introducing the monosilane, the rare gas and hydrogen gas as the starting gases into the film-forming chamber.
In the above constitution, further, the metal element is the one for assisting the crystallization of silicon, and is one or more kinds of those selected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
In the above constitution, further, the rare gas element is one or more kinds of those selected from He, Ne, Ar, Kr and Xe.